The Digital Imaging and Communications in Medicine (DICOM) standard was created by the National Electrical Manufacturers Association (NEMA) to aid the storage, distribution and viewing of medical images, such as images acquired by Computed Tomography (CT), Magnetic Resonance Imaging (MRI) and ultrasound (US). In accordance with the DICOM standard such images are normally stored in Composite Information Object Instances. A Composite Information Object Instance is a record which in one form contains image data and a header which contains attributes of the image data, such as the patient's name, the type of scan and image dimensions. A hospital will normally acquire and store a large number of medical images for different patients and indeed sometimes for individual patients at any one time and perhaps also at different times. For example images may be acquired from a patient during a first consultation by way of plural different modalities, i.e. types of equipment, and further images may be acquired from the same patient during subsequent consultations by way of plural different modalities with the actual modalities used perhaps changing from consultation to consultation. The DICOM standard therefore provides for the storage of attributes of the acquired medical images to impart a structure to the stored medical images and thereby allow for subsequent use of the stored medical images. Such use may include transmission to and analysis at a remote location with the attributes providing for proper analysis with regards to the identification of the patient, date of acquisition, modality of acquisition and the like. Analysis might, for example, comprise the superimposition of images acquired by the same modality at different times or the superimposition of images acquired by different modalities.
FIG. 1 is a representation of DICOM compliant data structure containing medical images acquired from a patient. Medical images for the patient belong to one of two Studies with a Study comprising plural Series of Composite Information Object Instances, which each contain medical image data and which are logically related for the purpose of diagnosis. A Study is modality independent and may therefore comprise medical images that are created by a single modality, by multiple modalities or by multiple devices of the same modality. Each of the Studies in FIG. 1 comprises plural Series with each Series comprising at least one image sequence which in turn comprises at least one medical image. To meet the DICOM standard all medical images within a Series must be of the same modality. Notionally an image sequence is often considered by the skilled reader as a volume. The DICOM standard thus defines the structure of the image data. However the DICOM standard incompletely defines the separation of the image data into volumes within a Series. The introduction to the DICOM standard in 2005 of multi-frame CT, MRI and PET Objects went some way to characterising volumes by means of explicitly defined dimensions, stacks based on position and indices of temporal position. Nevertheless such recently introduced structures are still insufficient to identify when an object changes orientation within a multi-frame object, for example if a patient moves during Functional MRI. Furthermore Studies complying with pre-2005 versions of the DICOM require considerable guesswork of a receiving application in respect of available descriptive, positioning and temporal attributes of a Series to determine what images constitute a set of images, a set of single images or a volume. As an alternative to guesswork the receiving application requires knowledge of the intentions of the creator of the problematic set of images, set of single images or volume, i.e. information outside the scope of the DICOM standard.
The DICOM standard provides a Frame of Reference Information Entity which is related to at least one Series. A Frame of Reference Information Entity (IE) identifies the coordinate system that conveys spatial information of Composite Information Object Instances (i.e. medical image objects) in a Series. Frame of Reference IEs provide the ability to spatially relate multiple Series to each other. Returning to FIG. 1, the data structure 10 comprises a first Frame of Reference IE 12 which is related to Series 1 of Study 1, a second Frame of Reference IE 14 which is related to Series 2 of Study 1, a third Frame of Reference IE 16 which is related to Series 3 of Study and a fourth Frame of Reference IE 18 which is related to Series1 and Series 2 of Study 2. A Frame of Reference IE designates a Reference Coordinate System (RCS), which is the chosen origin, orientation and spatial scale of a medical image in Cartesian space. In addition the DICOM standard provides a Spatial Registration Information Object Definition (IOD) and a Deformable Spatial Registration IOD. The Spatial Registration Information Object Definition (IOD) specifies the spatial relationship in a non-deformable fashion between Frames of Reference or between at least one volume of image data and a Frame of Reference. The Spatial Registration Information Object (IO) therefore specifies the spatial relationship between a source RCS and a registered or target RCS. The Deformable Spatial Registration Information Object (IO) operates in a similar fashion but provides for a deformable spatial relationship which is achieved by way of a deformation grid and perhaps also transformation matrices. The DICOM compliant data represented in FIG. 1 comprises one Spatial Registration IO (SR1) 20 which is related to Series 2 22 of Study 2 24.
Data within a Spatial Registration IO is encoded as a series of attributes. Registration may be provided for by one of two approaches. According to the first approach the Spatial Registration IO specifies a Frame of Reference as the target in one attribute and another Frame of Reference as the source in another attribute. According to the second approach the Spatial Registration IO specifies a Frame of Reference as the target in one attribute and at least one set of image data as the source in another attribute. Irrespective of whether the first or second approach is used, the Spatial Registration IO also specifies a transformation between the source and the target in a further attribute.
Considering FIG. 1 further, if one wishes to store the transformation between Volume 1 26 of Series 1 28 of Study 1 30 and Volume 2 32 of Series 2 22 of Study 2 24 a Spatial Registration IO 20 is created having its target specified as the fourth Frame of Reference 18 which is related to Series 2 of Study 2. In addition the Spatial Registration IO 20 has its source specified either as the first Frame of Reference 12 which is related to Series 1 of Study 1 or as a reference to the image data in Volume 1 26 of Series 1 of Study 1. The desired transformation from Volume 1 of Series 1 of Study 1 to Volume 2 of Series 2 of Study 2 is specified as an attribute in the Spatial Registration IO in matrix form. The resulting Spatial Registration IO is stored with the other objects shown in FIG. 1. As can be seen from FIG. 1 there are plural Volumes in Series 2 of Study 2 and each Volume could be used to define the same transformation to the fourth Frame of Reference. Furthermore a Spatial Registration IO (SRj) can be defined in respect of each of the Volumes (i.e. V1, V2, V3) in Series 1 of Study 1 such that:
SRj: T1j, T2j, T3j . . .
for each Volume in Series 1 of Study 1 where Tij is the transformation from Volume i of Series 1 of Study 1 to Volume j of Series 2 of Study 2.
Several issues arise from the structure of the DICOM standard as described thus far. Firstly, which of the Spatial Registration IOs (i.e. as defined above by SRj) is used when one wishes to transform between an image related to the first Frame of Reference and an image related to the fourth Frame of Reference? Secondly, how does one represent a non-identity mapping between or amongst Volumes in the same Series? Thirdly, how does one represent the transformation between a Volume in one Series and an individual Volume in another Series where the other Series has multiple Volumes? The underlying problem is the DICOM standard provides for mapping between a set of images (or Volume) and a Frame of Reference with the Frame of Reference being operative at Series level and thus associated with plural Volumes within the Series. One is therefore unable to represent mapping between one Volume and another Volume.
The DICOM standard provides a structure which might be used to address this problem albeit in a fashion which creates further problems as will become apparent from the following description. The structure, however, does not make the intended purpose explicit. This means a receiving application needs to make an inference as to the intended purpose on the basis of the explicit DICOM data. Alternatively the receiving application requires knowledge of the intended purpose and thus relies on information outside the scope of the DICOM standard. Considering this DICOM structure further, volume level information may be included by way of the Common Instance Reference Module (CIR Module), which is present in all composite Information Object Definitions (IODs) and therefore in all Spatial Registration IODs. The CIR Module enables a DICOM Object to reference at least one instance of any other Object. The CIR Module can be configured to specify a target set of images whereby a Spatial Registration IO defines registration to a target Frame of Reference with the transformation being determined with respect to the set of images specified in the CIR Module.
Use of the CIR Module in the fashion described in the immediately preceding paragraph has several drawbacks. The DICOM standard does not make the presently described use of the CIR Module explicit. As mentioned above, this shortcoming could be addressed by the receiving application making an inference as to intended purpose or relying on knowledge of the intended purpose, i.e. information outside the scope of the DICOM standard. In addition the present use of the CIR Module does not provide for proper handling of multi-frame images. A multi-frame image is a DICOM Object containing multiple images which need not necessarily contain complete volumes but could instead contain partial volumes. The CIR Module does not provide for referencing of individual frames of multi-frame images with compliance with the DICOM standard allowing for referencing of either the whole or none of a multi-frame image. This shortcoming could be addressed by insertion of individual frame numbers, which is a solution, however, that fails to comply with the DICOM standard. The presently described use of the CIR Module has two further irresolvable shortcomings. Firstly where there are plural transformations to the target Frame of Reference one needs to select the appropriate one of the plural transformations for a subject image. Secondly it may in certain circumstances be difficult if not impossible to determine a required registration. The difficulty is demonstrated by reference to FIG. 2 which shows a registration tree 50 in which each circular node represents a volume and the object is to determine the registration transform from volume (a) 52 to volume (b) 54. It is straightforward to determine the registration transform where each node has its own Frame of Reference because one can check whether or not the Frames of Reference match as one moves along the branches of the registration tree. On the other hand difficulty arises if each node in FIG. 2 represents a set of images. This is because one must determine as each step is taken through the registration tree if the image sets are equal; in other words one must determine if the list of images one is looking for matches exactly the list of images found in the Spatial Registration IO (i.e. the list of images at a node in FIG. 2). Determining whether or not image sets are equal adds considerably to the computational burden. If the image sets are equal the registration transform can be determined readily enough. If, however, the image sets are not equal, e.g. an image is missing from one of the sets or there is a non-identity mapping between an image in one set and the corresponding image in the other set, one is much less readily able to determine the proper registration transform.
The present invention has been devised in the light of an appreciation of the aforementioned problems. It is therefore an object for the present invention to provide a data structure for linking a first set of image data to a second set of image data.
It is a further object for the present invention to provide an image data processor for linking a first set of image data to a second set of image data.
It is a yet further object for the present invention to provide a medium for storing data for access by an application program being executed on data processing apparatus, the medium comprising a data structure stored therein, the data structure being configured to provide for linking of one set of image data to another set of image data.